SMIP 1996 Seminar

The parking structure studied in this investigation is the first parking structure from which significant strong-motion data has been obtained during January 17, 1994 Northridge earthquake. Although the structure did not suffer significant damage, the study of the recorded motions was conducted to evaluate the seismic response of parking structures during strong ground shaking and the adequacy of the current seismic design provisions for such structures. An important element of this research project was the use of system identification of the recorded motions in the parking structure, in order to estimate normal modes of vibration excited in the structure during the Northridge Earthquake. These normal modes were then used to calibrate a detailed finite element model of the structure which, in turn, was used to carry out detailed seismic analyses of the structure. The analyses indicated that proper modeling of all the elements of the parking structure including soil flexibility led to reasonable prediction of the main dynamic response characteristics of the parking structure. From this, several design recommendations were proposed in this study to improve the current modeling techniques and the code design provisions of parking structures.

In the aftermath of the January 17,1994 Northridge earthquake hundreds of strong ground motion and building response accelerograms were retrieved from stations throughout the greater Los Angeles basin. Particularly important among the building response records were the data obtained from instrumented buildings which experienced relatively large ground motions. This paper provides a summary of the results obtained from an elaborate two-year project which included inspection of the buildings, damage assessment, performance evaluations. The forces, displacements, and dynamic characteristics interpreted from recorded data are contrasted with those suggested by building codes. Key response parameters and characteristics of each building are studied and where necessary observations are provided which may be used to improve future editions of the building codes.

The dramatic increase in strong ground motion recordings over the last several years has provided both the impetus and opportunity to empirically examine the seismic design criteria in both the UBC and NEHRP code provisions. In this project both spectral design shapes and the usefulness of two spectral anchors are investigated using a comprehensive strong motion database and updated empirical attenuation relations. For the shapes, the results suggest that both the UBC and NERHRP design spectra provide enveloping criteria (except for site D at short periods) including cases for sites within 10 km of the fault rupture surface. For the NEHRP design spectra, comparison of the Fa and Fv factors to those implied by a recently developed empirical attenuation relation suggest that the NEHRP Fa factors may reflect too little nonlinearity while the Fv factors may show too much nonlinearity. Comparisons of the code shapes to the results from probabilistic seismic hazard analyses indicate that the fixed UBC shape has a moderate tendency to under-predict amplitudes for T > 1 sec in places like San Francisco and Sacramento where large (M > 7) earthquakes dominate the hazard at these periods. The more flexible NEHRP shape avoids this problem, but requires the specification of two anchoring points.

The Northridge earthquake provided the first case since the 1971 San Fernando earthquake of effective recovery, processing, use and analysis of data from accelerographs installed in buildings to meet local code requirements. A large number of records were recovered from these accelerographs in an effort funded by the California Strong Motion Instrumentation Program and the National Science Foundation, with Agbabian Associates coordinating and managing field recovery. The records were valuable to assist post-earthquake structural evaluation and form an important complement to data from regular networks. Significant aspects of the code data recovery, processing and analysis are reviewed. Lessons learned from this experience with code instruments are reviewed to improve the success in future events. Recommendations include increasing the documentation of the location, orientation, and maintenance of code- required instruments, and shifting from film to digital recorders for new installations. Also, a goal of increased adoption of the Uniform Building Code (UBC) instrumentation code requirement by cities and counties is discussed. CSMIP will assist city and county code-instrument programs by providing technical and monitoring expertise.

Structural responses recorded during three recent earthquakes (1992 Landers and Big Bear and 1994 Northridge) were used to evaluate the performance of the retrofitted single column viaduct (Route 101215 Interchange in Colton). The column deformation experienced range from low in the Northridge earthquake up to about 70% yield deformation level during the Landers earthquake. These results were compared with the deformation-based design practice for column retrofit. Effect of foundation contribution was also compared with the design practice. Results from this study support the successful performance of other similarly retrofitted bridges during the Northridge earthquake.

This paper summarizes the key findings and recommendations from a Seismic Safety Commission publication titled Northridge Buildings Case Studies Project (SSC 94-06) and how it influenced the Commission's report titled Turning Loss to Gain (SSC 95-01). The paper also describes how the ground motions recorded during the 1994 Northridge Earthquake were characterized by engineers during short seismic evaluations of 25 buildings.

Strong-motion recordings have been obtained in several buildings damaged by earthquakes. For post-earthquake inspection of buildings, strong-motion records can provide important information on the integrity of the building structure. This paper examines records from two concrete and four steel buildings damaged in earthquakes. The characteristics of the building response that may be indicative of the structural damage are identified from the records. For these buildings, the fundamental period, the maximum drift between the roof and the base, and the maximum base shear are all higher than the corresponding design values. In addition, high-frequency spikes and highly nonlinear response can be seen in some of the records.

Contradictory recommendations have been made in the Interim Guidelines FEMA 267/ Aug 1995, and the SAC Technical Report. 95-04, as to the value of analysis in post-earthquake evaluation of WSMF buildings.

Analytical studies of the response of welded steel moment frames (WSMF) using data recorded at the base and up the height of the building have been shown to be very useful in defining the zone of the building where earthquake damage is most likely. When damage is found in the zone where analysis would have predicted, this zone must be extensively sampled. Effort expended in a random survey for earthquake-caused damage is not earthquake response related and is not cost-effective.

A random testing procedure is useful to determine the effectiveness of the original quality control procedure. An analysis based damage survey is more productive in finding if the site shaking caused earthquake damage.